Grinding heads fitted with expansible grinders conventionally comprise means for rotating the grinder and moving same axially, as well as a device for controlling the expansion of the grinder.
Expansible grinders usually comprise abrasive stones, or diamonds, disposed along the generatrices of a cylinder and able to move radially under the action of a cone fixed to an expansion rod. The expansion rod may be moved, for example, by the piston of a hydraulic ram. In arrangements of this type, the movements of the ram piston have been hitherto restricted by means of electric valves controlled by microcontacts, or by proximity detectors.